modulus, chemistry & microstructure of … chemistry and microstructure...corrosion &...

Corrosion & Prevention 2012 Paper 48.00 -

MODULUS, CHEMISTRY & MICROSTRUCTURE

OF ALUMINIUM - ZINC OFFSHORE ANODES

Russell Northey - Metallurgist

Cathodic Diecasting Pty Ltd, Queensland, Australia.

SUMMARY: This paper details the solidification characteristics that occur while increasing the

modulus of cast galvanic anodes. These galvanic anodes, specifically those of the Al-Zn-In-Si group,

are broken into three groups by their modulus value. These products are used in their “wrought” or “as-

cast” condition; hence microstructure is primarily controlled by modulus. The paper looks at chemical

composition ranges used for these products and their possible application as a secondary means to

control or achieve a more desired microstructure. The paper also questions the current sampling

methods for electrochemical testing in relation to heavy modulus (heavy duty) offshore anodes.

Keywords: Solidification, Modulus, Chemical Composition, Microstructure, Electrochemical Testing.

1. INTRODUCTION

Having 30 years experience in metal producing industries I have found electrochemical testing to be somewhat non-

representative of the product it reports on. I will endeavour to detail that statement within this paper. This paper is intended

to give corrosion engineering personnel a better insight into the solidification theory of these “heavy” galvanic anodes.

This paper focuses on the Aluminium-Zinc-Indium-Silicon alloy(s) as being the more commonly used for long term, or

heavy duty requirements in ocean water, as these are most commonly called up for manufacture and “performance testing”.

Most (if not all) of the information I find on electrochemical testing has resulted from the testing of small test bars which

may not be fully representative of the actual grain size and microstructure that exists in much larger galvanic anodes.

There exists a large range of chemistry values available when comparing global specifications on these materials which can

raise the question: “Should these be refined to incorporate anode modulus?” Modulus will be defined, in contrast to size or

weight, as a comparative value which can be used to group items of similarity. There are some very specific chemistry

values, within the broad range, that might hold the key to matching chemistry with modulus to control microstructure.

Definitions used throughout the paper should be viewed as follows:

Wrought Alloy As Cast: I use this term “wrought alloy” to define material initially cast (galvanic anodes) without any

subsequent treatments. “As cast” to its final shape by gravity or pressure die casting. These materials receive no liquid

treatment (prior to casting) that has any influence on microstructure, nor any heat treatment (after casting) – hence my use of

the term “as cast” microstructure. The word “wrought” has many and varied meanings and I would ask that its meaning

should not be argued in this paper – “as cast” will suffice. Galvanic anodes are used in their as-cast condition.

Modulus: The absolute value of a complex number – I will detail this later – it is used to determine (and compare)

solidification times by calculating a given volume of metal divided by available surface area. I acknowledge that this may

not be the same meaning the reader has for the word modulus; again I ask that this is not to be argued in this paper.

1.1 Casting Modulus – explained for the CP Engineer

Foundry method engineers are familiar with this term, as it is used to calculate solidification time.

Fact Number 1 – The amount of heat that must be removed from a casting to cause it to solidify is directly proportional to

the amount of superheating and the amount of metal in the casting, or the casting volume.

Note the use of the word volume (not weight) – volume being expressed in cubic centimetres.

Fact Number 2 – The ability to remove heat from a casting is directly related to the amount of surface area through which

the heat can be extracted and the insulating value of the mould. Surface Area being expressed in square centimetres.

The foundry engineer uses a simple formula: Modulus (cm) = Volume (cm3) ÷ Surface Area (cm2)

The centimetre value becomes significant when used as an absolute to compare one “shape” to “another” – I can’t stress the

importance of this enough that it is the modulus we should be comparing, not the weight.


1.2 The significance of Modulus and Galvanic Anodes.

Perhaps we should break the word microstructure into two distinct categories, Grain Size & Phases present. We all assume

Aluminium Galvanic Anodes to be of a uniform grain size? We all assume Aluminium Galvanic Anodes to be made up

entirely of only the alpha phase in the microstructure? If we were to cast small test specimens, and examine these, that is

exactly what we will find. If we were to cut and weigh these, and subject them to some form of impressed current test, we

would see remarkable results to verify our observations?

Galvanic anodes are chill cast in either cast iron or steel dies so the microstructure will be that of the test bars? I disagree.

The missing link in this area of research is solidification time, and the effect of recalescence of solidifying aluminium.

Large platform anodes made using cast iron (or steel) moulds have lengthy solidification times – therefore have varying

microstructures – larger grain size, grain growth, interdendritic segregation, and precipitation are all possible outcomes.

I present again the physical presence of the equation: Modulus = Volume ÷ Surface Area.

2. CALCULATION OF MODULUS FOR COMMON GALVANIC ANODE SHAPES.

Figure 1. A graphical representation of data is presented below for observation – familiar shapes for galvanic anodes:


Figure 1 – Geometrical representation of casting modulus. Note the DNV long tern testing procedure as follows:

Refer DNV-RP-B401 - 12.2.1 – Quote: Specimens for testing shall be cut from full scale anodes using the same type of raw

materials, smelting, and casting practices as for normal production. The net mass of the anode shall be minimum 30% of the

maximum anode net mass of anodes for which the documentation shall apply. Guidance Note: The performance of an anode

material may be affected by solidification and cooling such that specimens cut from smaller anodes, or separately cast

specimens, may not be fully representative for larger anodes. End Quote.

The representation in figure 1 is split up into three groups, for which I feel are relevant for microstructure comparison. Also

note that the “shapes” presented assume a minimal solid insert (core) the like of 50 x 10 Flat, or 24 Round. For the heavier

module anodes pipe inserts are very common to ensure that (even at 50% consumption) the anode has a very good surface

area. This tube insert adds surface area “available for cooling” and lowers the modulus, which in turn shortens the

solidification time. This paper is reporting worst case scenario as sometimes heavy blocks are used with flat or round bar.

Upon examination of the data above the reader should see some relationship between modulus and solidification time.

This is basically a proportional comparison where I have used the Modulus2 x 1.2 to arrive at a solidification time.

The data above has been calculated, and coagulated with the experience of a galvanic anode manufacturing team.

Note the Platform & Block anodes take 10 times the solidification time of the 15kg hull anode. Also note the very short

times of the small diameter round bar and ask oneself if the sampling methods for electrochemical testing are relevant?

There might be concerns about the complete homogeneity of a galvanic anode which takes 16 minutes to solidify.

Note we have a 420kg Platform Anode, which takes 16 minutes to solidify. (2500 x 250 x 250). Above that we have 142kg

pier which satisfies the 30% weight rule for DNV long term testing; however with a modulus of 2.1cm it will complete

solidification in 5 minutes, perhaps not representative at all.

Below the 420 kg platform we have a 145kg block which also satisfies the 30% weight rule, and it does represent (or

exceed) the same modulus.

Please observe the cm3 ÷ cm

2 = cm function.

3. MODULUS VS MICROSTRUCTURE

A summary of the three groups from Figure 1.

Group 1 – Products with a Modulus less than 1.0 cm – instant to very fast Solidification.

This group would include almost all pressure die-cast components, Bar Stock (Cast) up to 25mm Dia, and most “flat block”

anodes with a thickness up to 25mm thick.

Group 2 – Products with a Modulus > 1.0cm < 2.5cm – A slight to moderate delay in Solidification.

The larger array of “hand cast” anodes – your general purpose hull anodes, larger diameter shafts and rods (cast > 25mm

but less than 60mm Dia), most tank anodes would fall into this category.

Group 3 – The larger “heavy duty” platform anodes – Modulus between 2.5 and 4.5 cm.

The physical metallurgy involved, suggest larger modulus “Platform Anodes” and “Heavy Duty Blocks” are a real concern.

The following section of the paper will give a detailed description of the solidification process for each of the above three

groups. Included are some sketches of what I would predict the microstructures to be.


Group 1 Products with a Modulus less than 1.0 cm – instant to very fast Solidification

Metallurgical Prediction – These would have an even (equiaxed) grain size distributed throughout the section of the

component. There would be very little, if none at all, loss of homogeneity in the microstructure with no signs of a “second

phase” due to fast solidification.

I have noticed this term “second phase” used in corrosion technical papers on these alloys.

I will use it “generally” (as other authors do) as it is hard to define if the second phase is either:

A: Indium behaving as partly insoluble. The only element in these alloys which can be unstable in OES analysis is Indium,

which indicates limitations in solubility exist. Also, many corrosion papers refer to forming “second phase Indium” which

falls in line with my thoughts. The papers, unfortunately, do not suggest Indium levels with a given casting modulus. Indium

is the only element in the composition table for these Zn, In, Si alloys which has a different crystal structure. Indium being

body centered tetragonal where as Al, Zn, Si, Fe, and many others are face centered cubic – interesting, and hopefully

relevant. In my opinion, as I can’t find any text to prove this, Indium might sit in the atomic lattice as an interstitial (in

between spaces) where as the other elements Zn, Fe, Si etc would be substitutional.

B: Indium-Zinc or Zinc-Indium compound. Looking at the phase diagram for the Aluminium-Zinc system one would

assume all the Zinc would be in solution in the “alpha” phase, especially given the rate of cooling these smaller modulus

anodes undergo. I would not rule out that some Zinc may be present in this “beta” phase as Indium may have a better

affinity for being soluble in Zinc and this insoluble phase is actually a Zinc-Indium compound. SEM or TEN examinations

would have to be done to verify this. We now need to define what level of Indium is required for these anodes if we are

seeing this in the microstructure – as the alpha will be anodic to the beta. A reduction of anode properties can occur.

C: Iron and/or Silicon. I would assume that Iron, is in Solution in the Alpha phase (Either as Fe, Al6Fe, Al3Fe, or as an

Al12Fe3Si compound(s) and has not had time to segregate to the grain boundaries) – provided the Silicon has been adjusted

to provide this “shielding”. These elements (Silicon and Iron) would most likely be responsible for nucleation sites if not

visible.

An in-depth understanding of nucleation, dendritic growth, and solidification of this Aluminium-Zinc alloy system is

required to present arguments for or against my logic. Examination of the phase diagram shows us a very small range

between liquidus and solidus. If one were to examine the microstructures keep in mind constituents A & B (above) will look

somewhat “splattered” like spilt ink, whereas C (above) may have a definite shape, and possibly colour, about them.

Group 2 – Products with a Modulus >1.0cm <2.5cm – Moderate delay in Solidification.

Metallurgical Prediction – These products would see differences in grain size if sectioned carefully. A “chill crystal” or

equiaxed grain would be distributed on the surface of the die, as solidification there is almost immediate (to a depth 2mm to

5mm deep) as soon as pouring stops. Beyond this “skin effect” we might observe a larger grain size and possibly some non-

homogeneity, as the dendrite arms could tend to be richer in Aluminium. There might be the chance of the liquid (in the

later stages of solidification) increasing content of Indium and/or Zinc in the pools outside the dendrite bodies. This might

be visible on the light Microscope at 400x-1000x magnification if they become available outside their respective solubility

limits. This may start to occur with the larger modulus anodes, such as the 15kg hull anode. There may be the tendency for

Fe-Si to be visible in the microstructure if they have segregated into the final pools of liquid during solidification.

Authors Note: This may all seem to be standard notation on the subject; however, there are some unique properties that

need to be understood on how these Anode Chemistries differ to our conventional metallurgical understandings.

I will detail these how I understand the process:

Solid Pure Aluminium has 2.5 times the thermal conductivity of the conventional Aluminium Casting Alloys (4xxx and

6xxx Series) due to its single phase microstructure – in the solid state. This means we can deliver large amounts of energy

for very little raise in solid temperature. This is unique to pure Aluminium, not so in “eutectic” containing casting alloys.

Liquid Pure Aluminium is a very poor conductor of heat – which is also important to know. By being a poor conductor of

heat the temperature of the liquid can be raised a substantial amount in a short period of time, with a minimum amount of

energy input.

Aluminium Zinc Anodes also have that “single phase” up to 6% Zinc, which nearly all of our anode specifications involve.

When we have solidified a skin, all the way around the casting, we now have a high thermal conductor between the liquid

and the cast iron die where the energy must be transmitted to for solidification to continue. An insulator of high thermal

conductivity will now inhibit solidification. It literally inhibits the transfer of heat through to the die walls.


The solid Aluminium’s high thermal conductivity further reduces the castings ability to follow these modulus predictions

and the energy being expelled from the dendrite arms undergoing solidification is passed onto the remaining liquid pools –

in preference to the already solid material – it follows the path of least resistance. The liquid pools increase in temperature!

Whilst those of us metallurgically trained know this phenomenon as “latent heat of solidification” – this property is

“logarithmically worse” with these Aluminium Anode materials. In simple terms, we all know pure Aluminium takes

extensive amounts of energy to melt; it is that high value of energy which is stored in liquid Aluminium, and released upon

solidification. Controlling the solidification of this material is a difficult objective.

This release of energy – in the later stages of solidification – is what is responsible for a loss of homogeneity, (the same

chemistry in every grain of material), and a loss of uniform grain size, due to the latent heat available delaying the

completion of solidification. We call this recalescence. Dendrites are growing slowly, and too few of them, also pools of

inter-dendritic liquid are getting hotter, not cooler. In respect to Alloying Elements – Zinc and Indium– the re-heating of the

liquid pools also results in high purity dendrites of the parent metal (Aluminium) and higher concentrations of Zinc and

Indium ending up at the grain boundaries. In some cases the Zinc and Indium may have gone past their solubility limits and

may be seen as a distribution of the Beta Phase.

With respect to Iron and Silicon – whilst we may tend to think of these as nucleants in small modulus anodes I’m persuaded

to believe these are also prone (in the larger modulus anodes) to “floating around” in the liquid, not due to density

obviously, but more for their melting point. These intermetallic compounds would have melting points in excess of 1400oC,

much higher that the parent alpha phase at 640-650oC. These compounds are happy to “float around” in the pools as “solid

compounds”, until final solidification occurs, hence they are visible at the grain boundaries, sometimes referred to as

“second phase Silicon”. They are perhaps still in the same form as they were in the ingot throughout the melting, alloying,

and casting operation.

I’m hoping at this point to have demonstrated that a small value modulus may not show these differences in the

microstructure and grain size throughout a component. However, when we increase our solidification time from a slight

period of 15-20 seconds, to a period of a minute or more, we have something different. We may no longer have a

homogenous solid solution. This variation in microstructure and grain size must surely have a bearing on the

electrochemical properties of an anode over its intended life.

My concerns – especially in regards to DNV testing – are that where a larger grain size exists, a larger consumption rate can

exist, as a larger size grain breaks away, when sacrificial. Where differences in micro-chemistry exist, and a beta phase

appears (A, B or C), the alpha phase is anodic to the beta phase, and anode consumption will increase. I believe this can

also reduce potential, as is the case with Iron and Copper in excess levels.

Where intermetallic compounds can combine, and find each other to form a larger particle, this particle causes the parent

alpha grain to be anodic to it, and consumption increases.

The above two statements are explained in more detail in the next section.

Group 3 – The larger “heavy duty” platform anodes – Modulus between 2.5 and 4.5 cm.

Metallurgical Prediction - These products would see real differences in grain size and I would predict areas of a second

phase at the grain boundaries. A “chill crystal” or equiaxed grain would be distributed on the surface of the die, as

solidification there is almost immediate (to a depth 2mm to 5mm deep) as soon as pouring stops. In fact, during casting of

these larger anodes, solidification across the bottom face and up the sides of the die will be occurring as filling is

continuing. Beyond this “skin effect” we would observe a larger grain size and possibly some non-homogeneity, as the

dendrite arms would tend to be richer in Aluminium. There is a very predictable chance of the liquid (in the later stages of

solidification) increasing content of Indium and/or Zinc in the pools outside the dendrite bodies. This mechanism will cause

these “globules” to eventually represent the Beta phase, and may be interpreted as having precipitated as a solid solution. It

may be more likely that they have evolved from the liquid pools as either (or both) the Zinc and Indium have reached, and

exceeded their solubility limits. This should be visible on the light Microscope at 400x-1000x magnification. There would

be the tendency for the intermetallics (Fe-Si) to be visible in the microstructure as they have segregated into the final pools

of liquid during solidification.

This mechanism of solidification, and the problems of changes in solubility in the interdendritic liquid during the

solidification, is delayed enough during the casting of these larger anodes for this prediction to take place. Remember the

re-heat principal (recalescence) is running havoc here – the energy from the solidifying material is superheating the liquid

pools. Remember what I said earlier about the conductivity of the solid aluminium (alpha), it is inhibiting heat transfer


outward. Those of us whom have had a good look during casting see the liquid centre (where we add the top up metal) go

from silver, to pink to bright white. This pool could be approaching 900-950oC yet we initially cast the object at 750-780

oC.

We now have “galvanic grains” where the primary (Alpha) phase will be Anodic to the secondary Beta phase. I believe this

phenomenon to be what is referred to by the Corrosion Metallurgists as “Intrinsic Corrosion” when the product is in service.

In my terms the anode will be consumed faster than a 25mm test piece. I would suggest that a “globular” beta has originated

from the liquid; where as a more “rounded” beta is a true precipitation in solid solution.

I’m extremely concerned about the internal integrity (non-homogeneity of microstructure and grain size) of these heavy

modulus anodes – especially after presenting the solidification structure. What we have here is a magnified problem of

group two - logarithmically.

This is the main discussion - the invalidity of smaller test bars. To fully represent a product they need to be taken, in my

opinion, from a large blank, at various stages of a run. Making a special heat and casting on a cold die does not represent

the product using a true die temperature. This of course becomes a very costly exercise.

Figure 2 – Sketches to represent predicted microstructures – Groups 1 & 2

Figure 2 – Sketches to represent grain size, and second phase presence with varying solidification time.


Figure 3

Figure 3 – Sketches to represent grain size and microstructural constituents in large modulus galvanic anodes. The lower

portion of the sketch detailing the “beta” components which can increase “anode consumption rate” and lower potential.


Figure 4

Figure 4 – Timeline description of an aerial view of a 2400 x 350 x 350 galvanic Aluminium anode.


Figure 5

Figure 5 – Representation of “reversal of heat flow” – top, and recrystallisation in process – bottom.


Figure 6

Figure 6 – section cut microstructures.

4. CHEMISTRY SPECIFIC TO MODULUS TO CONTROL MICROSTRUCTURE

After reviewing the many different proprietary specifications (by manufacturers), and end user specifications (equipment

owners), as well as a number of organisational standards, there is a large number of very similar specifications, in short

everyone “seems” to be doing a similar thing, however there is one strange trend: People (the three groups listed above) are

either quite specific with an elements range, Or, they list a specification that is very, very wide – with no explanation of why

it is so wide. This puzzles me as I’d not seen it in any other industries.

Table 1 – Review of chemical composition limits for large modulus anodes.

Zinc Indium Iron Silicon Titanium

DNV 2.5 – 5.75 % 0.015 - 0.040 % < 0.09 < 0.12 N/A

Specific 4.75 – 5.75 % 0.015 – 0.025 % < 0.07 < 0.10 < 0.025


Zinc and Indium

Note the wide range presented in the DNV specification. One can only assume that they are offering a “blanket” type

covering specification yet there is no guideline to particular casting modulus. I look forward to possible publication,

perhaps it isn’t available now because the detail of solidification hasn’t been looked at in the detail I present here?

Also note a very specific range I have observed from browsing several large anode producers specifications. A trend points

to these more specific limits, Zinc at the high end, Indium at the lower end, perhaps this is ideal for these larger anodes?

Iron & Silicon

The author feels a global agreement is needed on the Iron / Silicon rule, if there is such a rule? All the specifications I have

searched leave this area to trade secret. Perhaps there is no secret? I would suggest from my experience that the DNV short

term test does not have the “resolution” to give a definite result due to using a small size bar and only a gram of weight loss.

Primary Aluminium ingot always has a lower residual of Silicon than Iron, usually half, however all specifications leave a

hint that it should be added “to some rule of thumb” in order to be of greater amount than the Iron. Some specifications do

stipulate a minimum and maximum for Silicon, which is interpreted as there is a reason for it. It would benefit everyone if

we are to publish the benefit from the Silicon / Iron ratio thus remove the “less than or equal to” which leaves one guessing.

Do we agree that Si = (Fe > 0.06 x 1.5) – therefore Fe at 0.08 should have Si at 0.03; Fe at 0.12 should have Si at 0.09?

Titanium – As a Grain Refiner

Upon further browsing of global specifications one does note that some manufacturers, and some standards, do use

Titanium as a grain refiner. With knowledge of solidification of these large module anodes this is a great way to help

control grain size, microstructure, and inevitably promote uniform consumption rate – even if it does cost some capacity.

Adding nuclei at a rate of about 200-250 parts per million is a very, very advantageous method.

The DNV specification appears to have no acknowledgement of any grain refiner, specifically Titanium.

The use of a grain refiner does then involve (a purpose addition) in order to “provide a treatment to the liquid alloy” such

that it will influence the solidification structure. It moves one step away from a wrought “as cast” galvanic anode.

Figure 7

Figure 7 – Representation of the influence of Titanium on the cooling curve of large Aluminium galvanic anodes.


5. CONCLUSIONS

Hopefully, after reading through this paper, I’m hopeful the C.P. engineer is now a little more knowledgeable on the casting

process, and the mechanism involved which arrives at a given microstructure for these galvanic anodes.

I stress strongly, very strongly, the DNV short term test (or any other small sample test) is not representative of the heavy

duty, long term Aluminium galvanic anode. It is a fragile test for a very rugged component. It lacks resolution as it is

conducted with a very small test bar, and relies on mathematical conformance of a very small loss of sample material.

It does however produce the closed circuit potential everyone wants to see, however this also could be using a

microstructure not representative of the product of which it is intended to report on.

The necessity for repetition is not required once the alloy characteristics are defined. Analysis is an instant test.

The best test, if purchasers want to observe a manufacturers process, is OES (Optical Emission Spectroscopy) on a furnace

“bath” sample immediately prior to casting, and observation of the casting process itself. A pre-tap sample is that which is

representative of the chemistry of the anode(s) about to be cast – this is the testing area.

In regards to chemical specifications, for everyone’s benefit, I would like to see limits refined for “large” anodes.

Zinc – narrow the range to 4.75 – 5.75 % to ensure adequate capacity. Provided we can prove this is correct.

Indium – specifically 0.015 – 0.025 %, if excess is undesirable then lock it in. Again, we need proof that this is best

practice, and that excess indium can be detrimental to the anodes performance over time.

Silicon – define what is required, either a given range or a footnote to detail what is expected in relation to Iron.

There seems to be statement that the lower the Iron the better things perform, however, large quantities of high purity

Aluminium with Iron below 0.06% can’t always be obtained at short notice. If we have Iron in excess of 0.06 then we need

an international footnote on what Silicon should be used – this should be documented correctly.

Titanium – the advantage of the grain refiner Vs the anode properties needs to be distributed, to benefit everyone.

This paper is by no means definitive, except I have tried to define the grey areas in our manufacturing documentation.

I would welcome anyone wanting to assist in finalising these unknown variables. It benefits everyone.

6. ACKNOWLEDGMENTS

My educational mentors – without them I would have no presence in physical metallurgy.

I should also mention Robert Wlodawer whom wrote the book Directional Solidification of Steel Castings. It is his

instructions that most foundry engineers base their methods; fortunately I have had the opportunity to use this advice.

7. REFERENCES

Det Norske Veritas (2010) Recommended Practice DNV-RP-B401 – Cathodic Protection Design.

8. AUTHOR DETAILS

Russell Northey is the Metallurgist at Cathodic Diecasting Pty Ltd.

Started his trade in 1982 in the founding industry as a Jobbing

Moulder & Coremaker and progressed to obtain an Associate

Diploma in Applied Science – Metals Technology in 1990.

Has been involved in Melting & Casting operations of Iron, Steel &

Non-Ferrous alloys, for over 30 years.

Russell has been involved in making Galvanic Anodes since 2000.

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